Gravity Assist Podcast: Asteroid Hunting with Lindley Johnson

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Space.com, staff
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July 24, 2018 06:58am ET

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An artist's impression of a binary asteroid system making a close approach to Earth.

Credit: NASA

The Gravity Assist Podcast is hosted by NASA's Chief Scientist, Jim Green, who talks to some of the greatest planetary scientists on the planet, giving a guided tour through the Solar System and beyond in the process. This week, he's joined by Lindley Johnson, who is NASA's Planetary Defense Officer in charge of keeping Earth safe from the countless numbers of small bodies that cross our planet's orbit. As they say in this business, it's not a matter of if, it's only a matter of when.

You can listen to the full podcast here, or read the abridged transcript below.

Lindley Johnson: Well, you've got to find them first. You've got to be looking for them, find out what's out there or what may come close to Earth's orbit, and you need to find them well in advance of any close approach or impact to the Earth, because it will take us time — years, in fact — to be able to get out there and do something about them before they impact the Earth.

Jim Green: Is that because they're so small? I mean, they're so much smaller than planets and moons.

Lindley Johnson: Well, that certainly makes it more difficult to find them, but it's just simply the time that it takes to build a spacecraft to go out to the object and interact with it in some way to change its velocity. That's the main principle behind mitigating an impact — just simply changing the velocity of the object so that at the predicted impact point time, the object shows up late [and therefore misses us].

Now, if the object is small enough, just hitting it with a spacecraft — what we call a kinetic impactor — would be enough to change its velocity enough to slow it down. In fact, we're working on demonstration of that capability now. It's called the Double Asteroid Redirect Test (DART) and it is about to enter its full scale development phase.

Jim Green: Let's talk about DART a little bit. It will be our first attempt to intentionally change the orbit of an asteroid.

Lindley Johnson: That's right. We've chosen the target asteroid, Didymos, because Didymos is a binary asteroid. The primary object is about a half a mile across (780 metres) and it's orbited by a moon that is only about 170 meters (500 feet) across. So the DART spacecraft is going to do impact the moon and change its velocity and its orbit around the primary. We can observe that change from the ground, both optically and with radar. Of course, radar will give us more precise measurements and provide us the data that more precisely shows how much force we were able to impart on this moon. And since we're not changing the orbit of this whole system around the Sun, we're therefore not increasing the danger to Earth of this asteroid.

Radar observations of the asteroid Didymos and its small moon, which will be the target of NASA's forthcoming DART mission.

Credit: Arecibo Observatory

Jim Green: How are we able to see these relatively small objects? Aren't they very dark and black?

Lindley Johnson: Yes. There is certainly a population of these objects that are as dark as coal, so that makes them very difficult to see in the visible part of the spectrum, which is where we are mainly searching right now using ground-based telescopes. That's one reason why we would like to be able to search for them in the infrared part of the spectrum, because these objects absorb the heat from the Sun and then re-emit that heat as radiation that can then be detected in infrared light. The catch to that is that you need a spaced-based sensor to look for them, because the Earth's atmosphere blocks out the infrared.

Jim Green: We currently have one telescope in space that does look in the infrared.

Lindley Johnson: Yes, that's right. We have the Wide Field Infrared Survey Explorer (WISE), launched by NASA's Astrophysics Division, to build up an infrared map of the sky. It constantly imaged the sky to build up this map and we quickly figured out that with all the images that it was taking, we could look for an asteroid moving across the sky in those images. After the astrophysicists were done with it, the Planetary Defense Program here at NASA took over operations of the spacecraft [it became the NEOWISE mission] and we've made it not only a full-time asteroid hunter, but it characterizes them as well, figuring out their size.

Jim Green: How many near-Earth asteroids are out there?

Lindley Johnson: So far we have found over 18,000 asteroids of all sizes that come near Earth's orbit. But we think that's a very small part of the overall population. Right now, our task is to find asteroids larger than 140 meters (460 feet) across that come near Earth's orbit. We have several ground-based projects to do that. Our prediction is that there are 25,000 objects larger than 140 meters in size and so far we've only found 8,000 of those in the 20 years that we've been searching so far.

Jim Green: How many are really threats to Earth?

Lindley Johnson: We have to take several observations over a course of time to be able to determine their orbit and whether they're going to come close enough to Earth to be an impact hazard. Right now, with the known objects, there are none that have a significant possibility of impacting the Earth. There are several that come very close and if their orbits were to deviate from of our current predictions, they could become impact hazards. So, this subclass of near-Earth objects, which we call the potentially hazardous asteroids, have to be more closely monitored. We now have about 1,900 of those objects that we closely monitor.

Jim Green: Well, let's talk about one that flew by the Earth pretty recently. It's called Asteroid 2010 WC9. Tell me a little bit about it.

Lindley Johnson: From its designation, we know that this object was found in 2010. The WC9 tells us which month it was found it. It begins with a W, so we know that was the latter part of November of 2010, and then 9 is a sequential number [the ninth object discovered during that part of November 2010]. It's kind of a complicated designation system, but it works for the astronomers.

It was first observed by the Catalina Sky Survey in Arizona and just a few observations were taken of it during that time period. So, we didn't have a very good orbit for it at that time; we couldn't project the orbit out more than a year or so with any degree of certainty.

[But] we knew it would have another close approach with the Earth in May 2018, but we didn't know how close. In fact, the uncertainty in timespan of when the close approach would happen was 18 days. Moving at the speed that it does, it covers a lot of distance in 18 days. The Catalina Sky Survey, though, again reacquired this object on 8 May and started taking observations. The Minor Planet Center, where all the observations from around the world go to, was able to quickly correlate those observations with the orbital data that they had on this object. So, now we've greatly expanded the observation span on its object and can much more accurately predict its orbit.

A schematic of how NASA's DART mission will work, firing a kinetic impactor in the moon of Didymos, nicknamed 'Didymoon'.

Credit: NASA

Jim Green: One of the neat things about this o2010 WC9 is that it passed under the Earth and the Moon. I thought all these objects had pretty much Earth-like orbits close to the plane of the Solar System?

Lindley Johnson: The vast majority of them do. They're close to what we call the ecliptic, which is the orbital plane of the Earth in the Solar System, but there are quite a few that are more highly inclined orbits. Their orbital plane intersects the Earth's orbital plane at an angle that we call the inclination. For this asteroid, its orbital inclination is 18 degrees, which is fairly high. It actually goes in almost as close as Venus' orbit to the Sun and then goes back out to the Asteroid Belt beyond Mars. It probably originated there at some point following some gravitational encounter with Mars or Jupiter that put it into this more highly inclined orbit.

Jim Green: One of the other ways in which we not only characterize near-Earth asteroids, but get better orbital data, is through radar. How does this work?

Lindley Johnson: We first have to find them optically. We don't have powerful enough radar that can sweep the skies and find these objects. We have to know the orbit well enough so that we know when to expect the radar echo back when it bounces off of the object, because we've got to dig it out of the noise. If we didn't know where to look, we wouldn't see it at all. So, that's the first thing, and with enough energy bounced back off the object, we can do what they call radar imaging. It's a little different to optical imaging, but it's sort of the same principle, and we can get a good indication of the asteroid's size, much more precisely than we can optically, and we can also measure how fast it is rotating. The other thing that radar does for us is is it determines whether these objects are binaries. We know of several binary asteroids now — asteroids with a small moons circling them — and that's all been done by radar.

Jim Green: If they get close enough to the Earth and the radar hits them, you can actually see features on their surfaces.

Lindley Johnson: That's right, you can see craters or large boulders on their surface. It's really very interesting to see these radar images come back.

Jim Green: What kind of damage would we expect if one of these larger objects were to make it through the atmosphere and hit the Earth?

Lindley Johnson: It depends on the size of the object. This object, 2010 WC9, is estimated to be between about 50 and 120 meters in size. That could be a very damaging impact. For instance, the object believed to have created Meteor Crater in Arizona is estimated to have been only 50 meters in size. So, that's at the low end of our estimate of the size of this object. It would devastate a state-wide area if it were to impact.

NASA's WISE spacecraft has found most of the near-Earth asteroids larger than a kilometer, but many smaller and potentially hazardous objects remain undiscovered.

Credit: NASA/JPL-Caltech

Jim Green: What about objects half that size, maybe 20 meters? Do they make it to the surface?

Lindley Johnson: Pieces of them certainly will. It depends on how strongly they're composed. If they are an average, rocky asteroid, they will disintegrate in Earth's atmosphere. That, of course, recently happened in February 2013 over Chelyabinsk, Russia, when an object about 20 meters in size entered Earth's atmosphere at about 9am and detonated about 23 kilometers above the surface. The energy release was equivalent to about a half a megaton of energy.

Jim Green: The earlier the warning we have about an impact, the better. Are there other approaches that we're modeling or thinking about?

Lindley Johnson: There are at least a couple of other techniques that we think would be effective against an asteroid. One technique that we've done some modeling and development is called a gravity tractor. If you have enough time and the asteroid isn't too large, all you need to do is nestle up to it with a spacecraft and stand off it, and the mutual attraction between the spacecraft and the object, by gravity, will slowly tug that asteroid off its natural orbit into a new orbit. We would just be using nature's tug rope, gravity, to pull the asteroid into a more benign orbit. That would take several months to years to do that. But, the other nice thing about it is that we can put it into the exact orbit that we want to. We could tug on it a little while and then take measurements to see if we've changed the orbit enough, and if we need to do more, then we tug on it longer.

Jim Green: What do you think is the most common misconception that you see in the media or press when they talk about these asteroids?

Lindley Johnson: I think because it is something that really sparks the imagination and there have been movies about asteroid impacts, then every asteroid that we announce is going to have a close approach, sets off a concern that, "Oh, is this asteroid going to impact us?" I don't think they understand that once we have observations on the object and can establish its orbit, it's going to stay on that orbit. It is just not going to wander around the Solar System randomly. This is all controlled by the laws of nature, the laws of orbital mechanics, and once you've established a stable orbit, you can predict where that object is going to be many, many years in advance. With our current modeling capabilities, and if we have enough observations, we can confidently predict the orbit of an asteroid 100 years into the future.

NASA's first Planetary Defense Officer, Lindley Johnson, making a presentation to representatives from NASA and FEMA — the Federal Emergency Management Agency — on the dangers of asteroid impacts.

Credit: The Aerospace Corporation

Jim Green: One of the things that I ask each of my guests in this program is what was their gravity assist? In other words, what activity or event in their past propelled them forward and allowed them to become the scientist and engineer they are today. Lindley, what was your gravity assist?

Lindley Johnson: I've had so many of them in my career. I think the Apollo program, when I was growing up, got me interested in space and being part of the space program. That, in turn, got me into interested in being in the Air Force and being part of the Air Force's space program, where the opportunities I was given and the trust that was shown in me helped me to really develop my capabilities for program management and look forward into the future with these kinds of things. Then, coming to NASA, 15 years ago now, and being put in charge of this near-Earth object program, and working with the Solar System exploration missions have all given me the skills and the background to be NASA's first Planetary Defense officer.